The invention applies especially to the production of oxygen from atmospheric air and, in the rest of the description, reference will be made to this application as a preferred example.
The invention may be carried out with all types of adsorption cycles in which there is a pressure variation, for example with the following cycles:
So-called VSA (Vacuum Swing Adsorption) cycles, in which the adsorption takes place substantially at atmospheric pressure and the minimum pressure of the cycle is significantly below this atmospheric pressure and typically at about 250 to 400 mbar. These cycles are generally carried out by means of units consisting of three adsorbers. PA1 Transatmospheric cycles, called MPSA cycles, which differ from the previous ones by the fact that the adsorption takes place at a pressure substantially above atmospheric pressure and typically at about 1.3 to 2 bar. These cycles are generally carried out by means of units consisting of two adsorbers. PA1 So-called PSA (Pressure Swing Adsorption) cycles, in which the adsorption takes place at a pressure significantly above atmospheric pressure, typically at about 3 to 8 bar, while the minimum pressure of the cycle is substantially equal to atmospheric pressure. PA1 (a) From t=0 to T/3: a substantially isobaric production step at the high pressure P.sub.M of the cycle, which is close to atmospheric pressure. During this step, air is introduced into the adsorber via the valve V11 and flows from its inlet to its outlet, from where the oxygen produced leaves. Some of this oxygen is tapped off, in order to repressurize another adsorber during a repressurization step (c) described later, and the rest is sent for use at 8. PA1 (b) From T3 to 2T/3, the adsorber is depressurized or purged with a countercurrent by means of the vacuum pump 3, until the low pressure P.sub.m of the cycle is reached, this typically being around 0.25 to 0.40 bar. PA1 (c) From 2T/3 to T, the adsorber is repressurized with a countercurrent up to the pressure P.sub.M by the production oxygen coming from another adsorber in the adsorption step (a). PA1 the duration of the intermediate operation is at most equal to one third, and preferably between 1/3 and 1/50th of the shorter of the steps of the cycle that it connects; PA1 one of the spaces is a volume of gas mixture to be separated, typically the ambient air; PA1 at least one of the spaces is a gas storage tank; PA1 at least one of the spaces is a first adsorber which, during the said intermediate operation, communicates with the machine via one of its ends; PA1 during the intermediate operation, the first adsorber is also brought into communication with a third space via its other end; PA1 the third space is another adsorber which is at a pressure different from that of the first adsorber; PA1 the machine is an air compressor or blower, or a vacuum pump, with a single function; PA1 the machine is designed to operate as an air compressor or as a vacuum pump, depending on the steps of the cycle; PA1 the switching takes place by closing a first two-way valve and opening a second two-way valve, and the intermediate operation takes place by opening the second two-way valve before closing the first two-way valve; PA1 the switching takes place by closing a first way of a three-way valve and opening a second way of this three-way valve, the third way of this three-way valve being open, and the intermediate operation taking place by opening the second way of the three-way valve before closing the first way. PA1 the selective connection means comprise two two-way valves and the control means are designed to open the two two-way valves at the predetermined instants simultaneously; PA1 the selective connection means comprise a three-way valve and the control means are designed to open the three ways of this three-way valve simultaneously.
In the rest of the description, the acronym PSA will be used as a generic term for all these cycles.
Moreover, the pressures indicated are absolute pressures.
One of the means of reducing the cost of producing oxygen by PSA is to substantially decrease the capital investment, while keeping the energy consumption constant.
Reducing the cycle time falls within this scheme when the system in question allows the performance to be maintained despite more rapid steps. In practice, such a reduction consists in improving the kinetics of the adsorbents proportionately, in maintaining the head losses at their previous level, and preventing any problems of attrition of the adsorbent particles.
Horizontal flow through beds of adsorbents, coupled with the use of adsorbents of small particle size, allows most of these problems to be solved, and recent years have seen an increase in industrial units of this type.
However, it turns out that the cycles used at the present time, these mostly being directly derived from the cycles of longer duration used previously, are in fact penalized by the operation of the machines (air compressor or blower, vacuum pump) during the transient phases corresponding to the transition from one step to the next.
The reason for this is that, as will be shown later, the additional energy consumption associated with these transient phases is low in the case of conventional cycle times, but becomes significant in the case of short cycles.
A first example of these phenomena will be explained with regard to FIG. 1 in the appended drawings, which shows diagrammatically an example of a PSA plant for producing oxygen from atmospheric air.
This plant comprises: a blower 1; three adsorbers A1 to A3; a line 2 for feeding air to the adsorbers, which connects the output side of the blower to the lower ends or inlets of the adsorbers via respective valves V11 to V13; a vacuum pump 3, the output side of which is connected to the ambient atmosphere; a discharge line 4 which connects the intake of the vacuum pump to the inlets of the adsorbers via respective valves V21 to V23; and an oxygen flow line 5 connected to the upper end or outlet of each adsorber via tap-offs in parallel, namely respective tap-offs 6-1 to 6-3 equipped with respective valves V31 to V33 for oxygen production and respective tap-offs 7-1 to 7-3 equipped with respective valves V41 to V43 for repressurizing the adsorbers. Moreover, the line 5 is connected to an oxygen consumption circuit shown diagrammatically at 8.
Moreover, the plant includes control, regulation and electrical-supply means, known per se and not shown, which are designed to carry out the cycle illustrated in FIG. 2.
FIG. 2 is a diagram which illustrates a typical adsorption cycle carried out by means of the plant in FIG. 1.
In FIG. 2, in which time t is plotted on the x-axis and absolute pressure P is plotted on the y-axis, the lines bearing arrows indicate the movements and destinations of the gas streams and, furthermore, the direction of flow through the adsorber--when an arrow is in the direction of increasing y-coordinates (upwards in the diagram), the stream through the adsorber is called a cocurrent stream. If the upwardly pointing arrow lies below the line indicating the pressure in the adsorber, the current enters the adsorber via the inlet end of the adsorber; if the upwardly pointing arrow lies above the line indicating the pressure, the stream leaves the adsorber via the outlet end of the adsorber, the inlet and outlet ends being respectively those of the gas to be treated and of the gas drawn off in the production phase; when an arrow is in the direction of decreasing y-coordinates (downwards in the diagram), the stream through the adsorber is called a countercurrent stream. If the downwardly pointing arrow lies below the line indicating the pressure in the adsorber, the stream leaves the adsorber via the inlet end of the adsorber; if the downwardly pointing arrow lies above the line indicating the pressure, the stream enters the adsorber via the outlet end of the adsorber, the inlet and outlet ends being always those of the gas to be treated and of the gas drawn off in the production phase.
The cycle in FIG. 2, the period T of which is approximately 270 s for example, essentially consists of three successive steps. The cycle will be described below for one adsorber, for example the adsorber A1. In the case of the other adsorbers, the cycle is derived therefrom by a time shift of T/3 and 2T/3 respectively, T denoting the total duration of the cycle.
Looking at the instantaneous energy consumption of the vacuum pump during one step, it may be seen that this consumption increases uniformly as the pressure in the adsorber on which this machine is acting drops below atmospheric, and is then followed by a substantial peak on going over to the following adsorber, which is at a high pressure.
The diagram in FIG. 3, in which time t is plotted on the x-axis and pressure P is plotted on the y-axis, illustrates this variation and allows it to be clearly understood. Thus, near the point where the vacuum pump switches from one adsorber to another, i.e. near the times T/3, 2T/3 and T, the actual curve C1 departs from the theoretical curve C2. More specifically, in this FIG. 3, the time x corresponds to the closure time of the valves V2i (V21 in the example) and the time y corresponds to the opening time of the valves V2(i+1) (V22 in the example). These times are about 0.5 to 2 seconds, depending on the size of the valves.
In practice, during the transient period x, the gas output coming from the adsorber at the end of purging is throttled and the vacuum pump, for a very short time, pumps only on the volume of the vacuum circuit. Since this volume is much smaller than the volume of the adsorbers, the internal pressure in this circuit rapidly drops. Thus, a pressure drop .DELTA.P of up to 100 mbar below the theoretical low pressure P.sub.m of the cycle has been observed. Since opening the vacuum valve of the following adsorber, which starts only when the vacuum valve of the first adsorber has been completely closed, is not instantaneous either, there is also throttling of the pumped output until the valve has been completely opened (time y).
It follows from this that three times per cycle, during all the transient periods such as (T/3)-x to (T/3)+y, the intake pressure of the vacuum pump is substantially lower than the theoretical pressure (corresponding to the pressure in the adsorbers, less the normal head losses of the vacuum circuit). This results in an additional energy consumption proportional at each instant to the difference (actual P-theoretical P) for the type of machine normally used in these processes, namely usually a Roots-type vacuum pump. This additional energy has been estimated to be approximately 1% of the normal pumping energy for a VSA-type cycle with a cycle time of 3.times.90 s, a theoretical low pressure of 0.35 bar, for a .DELTA.P peak of 100 mbar and valve operating times of 1 second.
When the same 3.times.15 s cycle is carried out, with suitable adsorbents and a suitable adsorber geometry, adsorbers approximately six times smaller than previously are used for the same production, but the other equipment (air blower, vacuum pump and valves) remains generally unchanged. In particular, nothing precludes the size of the valves V21, V22 and V23 being different from that of the valves used for the 3.times.90 s cycle.
The valve operating time remains unchanged and the phenomenon described above, with the low-pressure peak and the overconsumption peak, occurs again. However, because the adsorbers are of smaller size, the plant for the 3.times.15 s unit is more compact, the pipework is shorter and the volume of the vacuum circuit tends to be smaller. The effects described above therefore tend to be amplified.
Assuming even that they are identical, their relative importance is significantly more substantial in the short-cycle case. The period of overconsumption thus represents 2 s over 15 s, instead of 2 s over 90 s previously. With the same assumptions as previously, this additional energy expenditure may thus represent up to 8% of the energy consumption of the vacuum pump in the case of the 3.times.15 s cycle.
It may therefore be seen that the effect in question, although relatively secondary in the case of the usual cycles, becomes important in the case of short cycles, and that it is necessary to remedy this in order to improve the energy performance of the latter.
As will be seen later, a similar problem arises in many other types of PSA cycles during switching of the compression and/or suction machines from one adsorber to another, from an adsorber to atmosphere or from atmosphere to an adsorber.